DESCRIPTION: (verbatim) The successful regeneration of articular cartilage has not yet been accomplished. The central hypothesis of our efforts is that the recapitulation of embryonic events serves as conceptual guide for the development of successful tissue engineering strategies. During the previous funding cycle we characterized the elements required for articular cartilage regeneration: progenitor cells and the appropriate microenvironment that includes biological and mechanical components. We have already defined the role of Mesenchymal Progenitor Cells (MPCs) in articular cartilage regeneration with their ability to differentiate into bone and cartilage in a variety of experimental conditions both in vivo and in vitro. We have now identified unique biomaterials based on hyaluronic acid (HA), major component of embryonic extracellular matrix, that have proven to be excellent delivery vehicles for MPCs. This progress forms the basis of the central hypothesis that structures these investigations: "HA-based scaffolds functionally organize tissue differentiation by controlling the developmental disposition of MPCs through direct cell-scaffold interaction and the subsequent action of their breakdown products on these cells and their progeny." The long-term goal of these investigations is the development of clinically useful and reliable strategies for the functional and durable regeneration of articular cartilage. We will use a well-characterized rabbit osteochondral defect model to determine the optimal components necessary for successful tissue engineered regeneration of articular cartilage (Specific Aim 1). The regenerative tissue will be assessed by conventional histology, immunohistochemistry, and mechanical testing. The fate of the implanted cells and the sequence of early events that place in the regeneration process will be analyzed with marking experiments and in situ hybridization respectively. The direct transfer of the conclusions derived from these studies to a clinically relevant animal model (goat) in Specific Aim 2 will provide the basis for the development of a tissue engineering strategy for the long-term functional regeneration of articular cartilage.